1. Field
The present disclosure relates to a water-soluble polyamic acid, a method of preparing the same, a binder composition for a lithium battery including the same, and the lithium battery manufactured using the binder composition.
2. Description of the Related Art
Polyvinylidene difluoride (PVDF), styrene-butadiene rubber-carboxymethylcellulose (SBR-CMC), and the like, which are mainly used as a binder in the battery field, exhibit good binding properties and good binding efficiency when used in carbon anode materials. However, when silicon active materials are used as an anode material, it is difficult to use PVDF or SBR-CMC because silicon active materials undergo volumetric expansion and shrinkage during battery charging and discharging, which makes it difficult to maintain mechanical and physical properties and adhesive strength by using PVDF or SBR-CMC.
Currently, lithium polyacrylate (LiPAA) exhibits the best properties in the silicon active materials, but easily breaks and has low toughness. Thus, when LiPAA is bent in a cylindrical form, it becomes broken or cracked, which makes it suitable for use only in coin cells.
Polyimide binders are engineering materials with good physical properties, chemical resistance and thermal resistance. They are regarded as materials capable of withstanding thousands of cycles in batteries, particularly, secondary batteries for automobile vehicles.
However, while such polyimide binders exhibit largely increasing lifespan, lithium ions may be absorbed thereinto, causing an irreversible reaction, which makes it difficult to prevent a decrease in initial efficiency. In addition, polyimides are insoluble in water, and thus methods of dissolving polyimides in a desired solvent by linking various kinds of functional groups to branched polyimide chains are used. However, these methods are not suitable for use in water-soluble polyimides.
With regards to the water-soluble polyimides, a post-treatment process, e.g., imidization of polyamic acid used as a binder, which is a polyimide precursor through heat treatment, is most widely used method as far as manufacturing costs and processes and solubility are concerned. However, when an electrode is manufactured using such method, it is difficult to raise the temperature of an electrode plate up to 160° C. or higher due to oxidation of a copper (Cu) substrate, and thus, a polyimide binder exhibits a low curing rate. If the curing rate is low, carboxylic acid groups of polyamic acid directly bond to lithium ions, and thus an irreversible reaction occurs, which results in decreased initial efficiency. In addition, unstable amide bonds exist, which may adversely affect an extended battery lifespan.
As such, although polyimide binders have high adhesive strength and good mechanical and physical properties, such polyimide binders are not suitable for use in actual industries due to long-term reliability deterioration by unstable bonds occurring due to difficulties in low-temperature curing, a decrease in initial efficiency due to an irreversible reaction of lithium ions, insolubility in water, and the like.
Thus, there remains a need in binders that may prevent a decrease in initial efficiency of a lithium battery.